Electromagnetic waves in a far-infrared region from about 25 μm to 4 mm in wavelength are also referred to as terahertz waves and have both transmittance of a radio wave and straightness of light, and many substances have inherent peaks in absorption spectra in this region. Therefore, the electromagnetic waves are expected to be effective in identifying substances. However, conventionally, there have been no small and easy-to-use light sources for emitting light in this region, and a detector therefor needs to be cooled by liquid helium or the like and is therefore difficult to handle. Thus, the electromagnetic waves have been used only for limited research use.
In 1990s, light sources and detectors using femtosecond lasers, which have a small size and do not need to be cooled, were implemented, and research and development for implementation thereof have been eagerly performed. At present, general-purpose spectrometry devices based on time-domain spectroscopy are commercially available, and research and development are being performed for application to various fields such as security, biosensing, medical/pharmaceutical, industrial, and agricultural fields (for example, see NPL 1). Since about 2000, compact coherent light sources capable of tuning a frequency in a broad band have also been eagerly researched, which causes increase in power thereof (for example, see NPL 2). Furthermore, a technique for performing detection using a non-linear optical crystal at high SN has also been developed (for example, see NPL 3).
For industrial application, it is required to acquire an image of a specimen in many fields. As means for achieving this, there has conventionally been known a method of acquiring an image by placing a specimen on an XY stage and repeatedly measuring the specimen while moving the specimen by using a spectroscopic analysis device for point detection (for example, see NPL 1). Further, a method using a two-dimensional array far-infrared light detector (for example, see PTL 1) and a method of acquiring an image by using an electrooptical crystal and a two-dimensional array CCD camera for visible light (for example, see PTL 2) are also proposed. Furthermore, a method using a one-dimensional array far-infrared light detector is also proposed (for example, see NPL 4).